The present invention generally relates to the excavation of tunnels and more particularly, to a method of excavating a tunnel in hard bedrock without blasting.
A breaker or a boom type tunnel boring machine can be generally employed for excavating a tunnel in bedrock composed of soft rocks or rocks softer than intermediate hard rocks. Meanwhile, these machines cannot be used for excvating a tunnel in bedrock composed of hard rocks and thus, blasting is employed therefor. However, blasting has drawbacks in that vibrations and noises are produced and stones are blown away, thereby aggravating environmental conditions.
Thus, in order to eliminate the above described problems, there has been proposed a method of excavating hard bedrock without blasting as shown in FIGS. 1 and 2. In this known method, a plurality of bore-holes a are drilled on a facing of a tunnel so as to decrease the strength of the bedrock. Then, the bedrock is crushed by a breaker b. In this known method, a central portion of the tunnel can be excavated. However, this known method is disadvantageous in that, since a chisel of the breaker b is likely to slip circumferentially inwardly from the surface of a peripheral wall of the tunnel, it is difficult to finish the peripheral wall of the tunnel accurately. Furthermore, this known method has a disadvantage that since the breaker b is required to be directed upwardly in the case of excavation of an upper end portion of the tunnel, the striking efficiency of the breaker b drops excessively, thus resulting in poor excavation efficiency.
Meanwhile, in another prior art method, a plurality of holes c are bored on a face of a tunnel as shown in FIGS. 3 and 4. Then, an expansion agent including lime is filled into the holes c such that cracks d are produced, between adjacent ones of the holes c, in the bedrock. Thereafter, the bedrock is crushed for excavation by the breaker b. However, this prior art method has an inconvenience in that since a long time period is required for generating a predetermined expansion pressure upon curing of the expansion agent, the waiting time in excavation work increases when a bedrock portion to be excavated is limited in area as in the case of the tunnel.
Furthermore, in these known methods, the bedrock is crushed for excavation, mainly through shear fracture thereof, by the breaker b. It is to be noted that shear strength of hard rocks is approximately twice as large as their tensile strength. Thus, these known methods have such drawbacks including poor working efficiency as compared with that of methods based on tensile fracture.